ASSESSING PALEOCURRENT DIRECTION WITH BONE ORIENTATION AT WACO MAMMOTH NATIONAL MONUMENT, TEXAS, USA

Waco Mammoth National Monument (WACO) is a late Pleistocene lagerstätte in Central Texas, USA near the confluence of the Bosque and Brazos rivers. The lower fossiliferous layer, dated at 66.8+- 5.0 ka, preserves over 15 Columbian mammoths of varying levels of articulation that have been interpreted as a matrilineal nursery herd. WACO’s depositional environment and pre-depositional relationship with the Bosque and Brazos rivers are poorly understood. Faunal and sedimentological evidence supports the presence of water, but the existence, direction, and strength of a paleocurrent has not been demonstrated. We hypothesized that if a consistent paleocurrent was present at WACO, bones should be preferentially aligned with paleocurrent direction along their long axes, especially amongst disarticulated elements. Using ImageJ, we measured the angle and length of 317 skeletal elements on the original lower level quarry map. We converted the angle measurements to azimuths and used the “circular” package in R to create rose diagrams. We plotted rose diagrams of azimuths for 1) all elements, 2) elements over 20 cm, and 3) elements under 20 cm. We also conducted Rayleigh’s tests with each diagram to test for preferential orientation of elements. The samples of all elements and of elements under 20 cm demonstrated a weak but significant tendency toward Northeast-Southwest orientation (p = 0.0042 and p = 0.0159 respectively), but the sample of elements over 20 cm did not show significant orientation (p = 0.1187). These results indicate a Northeast-Southwest trending paleocurrent strong enough to influence fossil transport and deposition in small elements, but not large ones. We interpret this as evidence against an extremely low-energy depositional environment such as a pond, which would have no paleocurrent, and against a high-energy fluvial environment, which would have a strong enough paleocurrent to align larger bones. Rather, we propose a low-energy fluvial environment such as a tributary or overbank floodplain. Future work is needed to assess potential paleocurrent variation across space and time within WACO, which could provide further insight into the relationships between the Brazos and Bosque rivers, WACO's paleoenvironment, and the WACO mammoths.